Data communication systems such as, for example, fiber optic systems utilize optical receivers for receiving data transmitted over fiber optic cables. Such optical receivers additionally provide clock recovery, encoding, decoding, and data buffering. The optical receiver may be fabricated on a single semiconductor chip.
The optical receiver typically includes an analog receiver circuit which receives an input current signal from a diode disposed to receive the signals transmitted along the fiber optic cable. Typically, for digital transmission, the receiver includes a transimpedance preamplifier and a linear channel composed of one or more postamplifiers. The output of the linear channel is applied to a comparator decision circuit which generates a digital signal representing the received data. Typically, the preamplifier has an inherently wide noise bandwidth. Conventionally, the linear channel is utilized to filter this input noise. By selecting a particular bandwidth for the linear channel, this input noise can be suppressed. However, a problem exists in that the linear channel bandwidth will change considerably with manufacturing processes and temperature variations of the receiver. A bandwidth which is too large will allow spurious noise signals to pass through the linear channel causing data or phase errors. Conversely, a bandwidth which is too small will prevent data from being received by the receiver. Therefore, the linear channel ideally should be bandwidth calibrated for proper operation of the receiver circuit.
Although methods have been previously devised for calibrating the linear channel, such as or by external testing and trimming components, such methods by themselves have been deficient and do not provide automatic bandwidth control of the linear channel.
A need has thus arisen for a linear channel bandwidth calibration circuit for the automatic and accurate control of the bandwidth of the linear channel.